1851 Great Exhibition: Official Catalogue: Class X.: James Prescott Joule
440. JOULE, JAMES PRESCOTT, F.R.S., Acton Square, Salford — Inventor.
Electro-magnet, constructed of a plate of well-annealed wrought iron, tapered to the poles. The iron is rendered magnetic by transmitting the voltaic electricity through the bundle of copper wires (fifty yards long, and weighing one hundredweight) with which it is enveloped. Armature to the same.
Pair of tapered armatures, to concentrate the magnetic force when the electro-magnet is excited by a feeble voltaic current, and to direct the magnetic action to any required object.
Surface electro-magnet, consisting of a thick piece of wrought iron, enveloped by a bundle of copper wires. Armature to the same. A battery of moderate power produces such a powerful attraction between the electromagnet and its armature, that a weight of more than one ton has to be applied in order to draw them asunder.
[The peculiarity of the former electro-magnet, which is adapted for diamagnetic experiments, the magnetization of steel bars, etc., consists in the great comparative breadth of the iron core, by means of which the full effect of the coils of wire is secured, even in the case of their being removed to the distance of one or two inches from the surface of the iron. The form of iron adopted admits of a much greater quantity of electro-magnetic coils, and consequently of a much greater magnetic effect, than can be obtained from the usual cylindrical shape.
The peculiarity of the latter, or surface electro-magnet (which is the first of the kind ever constructed), consists in the comparatively great surface of contact which it presents to its armature. The principle of its construction, and of that of others of its class subsequently constructed by the exhibitor and other parties, is derived from the law of electro-magnetic action discovered by the exhibitor, viz., that the maximum attractive power of an electromagnet for its armature is about 300 lbs. for each square inch of tranverse section of the magnetic circuit.]